Onset and Growth of the Electron-Hole Liquid Phase in Germanium

Abstract

Under fast optical excitation and with a fast detector we were able to resolve in time the onset and growth of the electron-hole liquid phase Ge. We found that the onset of luminescence was delayed by 35 nsec. Thereafter the luminescence increased to a quasisteady state with a time constant of 550 nsec and subsequently decayed with a 35-jusec time constantIntensive study has been devoted to various properties of the electron-hole liquid phase that forms in Ge at low temperatures and high excitation levels. The present study is concerned with the onset and growth of the liquid phase using narrow optical pulses for excitation and a fast superconducting tunnel diode as a detector. With the exception of one brief report, 1 only the decay kinetics of the electron-hole droplets (EHD) have been studied 2 " 5 in experiments in which the optical excitation was produced by relatively wide laser pulses (^ 0.1 Msec) and the detection rise time was of the order of 1 Msec.In this Letter, we wish to present experimental results obtained using a Ge crystal optically excited by 15-nsec pulses from a cavity-dumped argon laser delivering 1 [iJ/pulse at a wavelength of 514.5 nm and a repetition rate of 2 C 5 kHz. The novel feature in this experiment is the detector, which was a Pb-PbO-Pb tunnel diode (Pb diode for short) with a rise time of about 10 nsec. This fast detector enabled us to study experimentally for the first time the sequence of events that take place in the crystal from the time of incidence of the optical pulse to the complete decay of all excitations.We used high-purity Ge single crystals with resistivity ~ 50 SI cm at room temperature and dislocation density of ^ 500 cm" 2 . The crystal was cut in the form of a parallelepiped, 3.5x3.0x0.5 cm 3 , with sides parallel to the principal axes [100], [010], and [00l]. After cutting, the crystal was carefully etched in CP4 and the two large opposite faces were Syton polished to an optically smooth surface which is damage and strain free as well. On one of the large faces, we prepared two Pb diodes, 0.5x0.5 mm 2 and ~500-A film thickness, as shown in Fig. 1. The laser beam was incident on the opposite face.A magnetic field of ~ 100 G is applied parallel to the [OlO] direction to quench the dc Josephson current of the detector 0 The diode is then biased at a voltage V B < 2A/e, where 2A is the superconducting energy gap (=2.75 meV for Pb). Incident radiation, either photons or phonons, of frequency v such that hv > 2A is absorbed in the superconducting films by breaking Cooper pairs. This increases the excited quasiparticle population and hence the tunneling current. The detected signal is a voltage proportional to the rate of absorption of energetic quanta. With the point of incidence indicated by the arrow in Fig. 1, the phonon signal was sufficiently small compared to the photon signal that the full time evolution of the latter was accurately obtained. We use a sampling scope to obtain a plot of the signal versus time.The experimental signal is shown in Fig. 2(a) on a tim...